It took me several months and a lot of head ache, but now my configuration works fine:

Previous configuration:

WD-TV Live with a D-Link USB WLAN-connection (recommended stick) connected to the router 1 via WLAN (about 30 meters and a concrete wall in between), the router 1 connected to a combined router/DSL-modem 2 via LAN cable (1 meter), disks connected to the router 1 locally near the router 1.

problem: movies freezing after a few minutes

New configuration:

WD-TV Live connected directly to the router 1 (1 meter), the router 1 connected to the router 2/modem via PowerLan (30 meters and a concrete wall), disks and router 1 moved 30 meters to the WD-TV Live and the TV.

I hope this helps, I saw a lot of WLan problems in the forum. I would just find a way to avoid WLan.